The present invention relates to an improvement in a continuous annealing method for obtaining cold reduced steel sheets suitable for general press forming, and more particularly, to such method using a heating cycle based on the most suitable starting temperature of rapid cooling in order to impart to the steel sheet, good press-formability, i.e. higher work hardening index and superior aging resistance.
The continuous annealing is characterized most in its extremely high productivity as compared with the so-called batch type annealing. This is due to its shorter time spent in the furnace. However, the method cannot help being defective in aging resistance and in press formability for that very reason. Thus, it was the general practice in the prior art, that this method was not employed as an annealing process for steel sheet used for drawing or press forming. However, it is true that such a continuous annealing process can embody the higher productivity and superior uniformity of the quality of steel sheet which are not obtainable through the conventional batch type annealing. Thus, if the continuous annealing process can be effectively employed to obtain steel sheets usable for drawing purposes, highly usable and unexpected commercial value can be effectively obtained.
Various proposals have heretofore been made regarding the aforementioned point. The basic and representative example is disclosed in U.S. Pat. No. 2,832,711, from which are derived many other examples and improvements. The heating cycle disclosed in U.S. Pat. No. 2,832,711 is such that the cold reduced steel strip is heated up to 1,250.degree.-1,300.degree. F. (677.degree.-705.degree. C.), rapidly cooled to below 1,000.degree. F. (538.degree. C.), and then maintained at 800.degree. to 1,000.degree. F. (427.degree.-538.degree. C.) for more than 30 seconds. The steel sheet thus obtained was, however, undeniably inferior to the conventional steel sheet obtained through the batch type annealing process in its press formability, particularly in work hardening index, i.e. "n" value. This was attributable to the inappropriateness of starting temperature of rapid cooling. Various improvements made on this method also presented similar defects. Thus, the practical application of the process in actual production of steel sheets has been curbed.